1. Field of the Invention
This invention relates to silicone rubber molded products and a method for their production. More particularly, this invention relates to silicone rubber molded products that are unitary articles containing a cured conductive silicone rubber component containing carbon black and a cured silicone rubber component containing substantially no carbon black.
2. Description of the Prior Art
Molded silicone rubber articles in which the cured body has a portion of silicone rubber containing carbon black and a portion of silicone rubber containing substantially no carbon black are widely used as two-color keyboard pads, zebra-type connectors, and electrical contacts in various types of electrical and electronic parts.
Japanese Patent Publication No. 56(1981)-41417 published Sept. 28, 1981 in the names of Sado, et al. describes a method for producing the aforementioned molded silicone rubber articles. An unvulcanized carbon black-containing silicone rubber is combined with an organic peroxide and then molded at an elevated temperature under an elevated pressure to obtain the conductive silicone rubber cured, i.e., vulcanized, component. An unvulcanized insulating silicone rubber containing substantially no carbon black is combined with an organic peroxide having an activation energy of at least 33 kcal/mole and the resulting mixture, together with the above-mentioned cured conductive component, is molded at an elevated temperature under an elevated pressure to manufacture a unified silicone rubber molded article containing the conductive silicone rubber cured component and the insulating silicone rubber cured component.
The above-mentioned method tends to suffer from an inhibition of vulcanization of the insulating rubber component by the organic peroxide due to the carbon black present in the conductive silicone rubber. For this reason, the organic peroxide to be used should have an activation energy of at least 33 kcal/mole and is limited to compounds such as dicumyl peroxide, di-t-butyl peroxide, and 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane. However, even when the conductive rubber component is vulcanized in the presence of an organoperoxide which does not tend to exhibit vulcanization inhibition by the carbon black, vulcanization is still partially suppressed. In particular, the surface of the cured article is insufficiently cured. Due to this, the above-mentioned method exhibits drawbacks such as requiring a long vulcanization time at high temperatures during the molding process in order to manufacture a satisfactorily cured product.
U.S. patent application Ser. No. 447,588, filed on Dec. 7, 1982, discloses unitary molded silicone rubber articles similar to those disclosed in the aforementioned Japanese Patent Publication and which are useful as electrical contacts. The articles consist of a first portion of conductive silicone rubber containing from 5 to 75% by weight of carbon black and a second portion of insulating silicone rubber containing substantially no carbon black. The second portion of silicone rubber is cured in the presence of the previously cured first portion using an addition reaction and a platinum catalyst, and the first and second portions are united together to form a unitary article.
The method disclosed in this U.S. patent application avoids the disadvantages inherent in the method disclosed in the aforementioned Japanese Patent Publication by employing an insulating silicone rubber component that is cured using a platinum catalyst. The conductive rubber component can be cured using an organic peroxide or the same method employed to cure the insulating rubber component.